The present invention relates to a controllable diffraction device for a light modulator device used for a display for the presentation of two- and/or three-dimensional image contents or image sequences. Thereby, the controllable diffraction device comprises at least two substrates, at least one electrode on each of said substrates facing each other, and liquid crystals forming at least one liquid crystal layer arranged between said electrodes on said substrates, whereby the orientation of the liquid crystals is controllable by a voltage supplied to the electrodes and the liquid crystal layer is provided on at least one alignment layer arranged on at least one electrode on said substrates.
A controllable diffraction device is for example described in WO 2010/149587 A2. This reference provides a light modulation device comprising a (spatial) light modulator, a controller and a diffraction device, which has a variable diffracting structure. Thereby, the phase and/or the amplitude of a light wave field, which is substantially collimated, can be varied by means of the light modulated depending on the location of the light modulator. The (spatial) light modulator is controlled by the control device. Moreover, the light wave field varied by the (spatial) light modulator can be diffracted in a variable and predetermined manner by the diffraction device having the variable diffracting structure. In order to provide a holographic reconstruction for an observer, the position of the observer is tracked and the variable diffraction unit deflects the light wave field according to the observer's position.
A controllable diffraction device as discussed above may be formed as a liquid crystal (LC) device and may be considered as a liquid crystal grating (LCG).
Thereby, the electrodes of said controllable diffraction device are controlled such that an electric field distribution is obtained in the diffraction device, which realizes at least regionally a saw-tooth shaped refractive index distribution with a presettable periodicity. This can be achieved for example in that in respect of one direction the neighbouring electrodes are supplied with different electric voltages. Consequently, an electric field is generated between two substrates of the diffraction device, which influences the orientation of liquid crystals arranged between the substrates such that a saw-tooth shaped phase profile is obtained providing a saw-tooth shaped refractive index distribution.
Thereby, the switching of orientation of the liquid crystals depends amongst other factors on the alignment layer and the applied electric field.
For example in the known Electronically Controlled Birefringence (ECB) mode, the LC molecules in a LC device are oriented in the absence of an electrical field by an angle, which is typically in the range of 3° to 8°, to the substrates and to the electrodes on these substrates due to surface alignment dominated by interactions between the alignment layer and the liquid crystals.
Moreover, LC molecules with a positive dielectric anisotropy are used for this mode. If the field is switched on, a force acts on the LC molecules to orient them parallel to the field that means rather perpendicular to the substrates.
The resulting orientation is achieved by a minimum of the sum of surface anchoring energy, elastic energy of the LC and energy by interaction with the field.
As a further example, LC molecules with negative dielectric anisotropy are used for a LC device based on the Vertical Aligned (VA) mode. Thereby, the LC molecules are oriented by an angle, which is typically around 82° to 87°, to the substrates due to the interaction with the alignment layer, and a force to orient them rather parallel to the substrate acts on them if an electric field is applied.
In the ECB mode, two possible rotation directions—either clockwise or counter-clockwise—for the LC molecules in the applied field are possible if no angular pre-orientation is provided.
However, said arrangement of the LC molecules parallel to the substrate would result in domain formation, whereby in some domains LC molecules orient clockwise and in other domains LC molecules orient counter-clockwise. This effect leads to disclinations, which have negative influence to the optical performance of the LC device.
In order to avoid domain formation, the LC molecules close to the alignment layer may be pre-oriented by a pre-tilt angle relative to the alignment layer. For example, as mentioned above for the ECB mode, LC molecules oriented by 8° clockwise due to the surface alignment have only a 82° clockwise rotation angle in order to be parallel to the field but would need a 98° degree counter-clockwise rotation angle. As a consequence, clockwise rotation is energetically preferred and all molecules rotate in the same sense.
This pre-tilt can also be induced by rubbing. For example rubbing direction from left to right may cause a counterclockwise pre-tilt and rubbing from right to left causes a clockwise pre-tilt. The amount of the pre-tilt angle depends also on the type of polyimide material. For example, LC devices with out-of-plane rotation such as Twisted Nematic (TN) devices or ECB devices provide pre-tilt angles in the range of 3° to 8°. Thereby, the rubbing strength may be a parameter to define the value of the pre-tilt angle.
Furthermore, also other types of alignment exist, for example photo-alignment by use of polarized UV-Light. Thereby, the irradiation time or the heating temperatures may be parameters to define the value of the pre-tilt angle. Moreover, a procedure typical for inorganic alignment layers is evaporation of the layer at oblique incidence.
Treatment of the alignment layer for example by one of the above mentioned methods thus creates a preferred direction for orientation of the LC molecules close to the alignment layer. The other LC molecules further away from the alignment layer will also preferably orient in the same direction due to elastic forces among the LC molecules. For example in case of mechanical rubbing this preferred direction is almost parallel to the rubbing direction. One possible explanation for this effect is that polymer side chains of the alignment layer material are oriented by the rubbing procedure and the LC molecules preferably orient parallel to these side-chains.
In an electrical field because of their dielectric anisotropy LC molecules tend to orient in the field. If their dielectric anisotropy is positive they preferably orient parallel to the field, if it is negative they orient perpendicular to the field.
Moreover, the pre-tilt angle influences the viewing angle performance in some LC devices. In a VA liquid crystal display, for example, there is typically a Multi-domain vertical alignment (MVA) configuration that means there are different domains within one pixel having the same amount of polar pre-tilt angle but different alignment direction, which corresponds to a azimuthal angle of LC orientation.
For these types of saw-tooth gratings, it is not possible to write a perfect prism type (saw-tooth shaped) phase profiles. Due to electrode structures and smoothing by the elastic response of the LC molecules, falling edges of the phase profile exist reducing diffraction efficiency of the deflection grating structure in the desired order. It is thus desired to keep the falling edge of the prism as small as possible.
For the use of such a deflection device for observer tracking it is desired to have a large angular deflection range. Especially it is desired to have a symmetrical tracking range. That means the deflection element should deflect for example to the right side and to the left side to approximately the same angle and with the same efficiency.